[ENHANCEMENT] Show the % of BCD keys mapped

[jcscottiii]

Work with @ddbeck to get the data. Since it is a burn down, we don't need to store this long term in our API. We can just grab the data from somewhere publicly available and show it on the dashboard.

[ddbeck]

I've filed web-platform-dx/web-features#2021 for web-features to make this info readily available. Would love to know if that would work for you, @jcscottiii. Thank you!